Add additional input directories to parse when generating documentation

[nkoenig]

This supports adding additional directories to documentation generation.

Usage example:

ign_create_docs(
  API_MAINPAGE_MD "${CMAKE_BINARY_DIR}/api.md"
  TUTORIALS_MAINPAGE_MD "${CMAKE_BINARY_DIR}/tutorials.md"
  ADDITIONAL_INPUT_DIRS "${CMAKE_SOURCE_DIR}/src/systems"
  TAGFILES
   "${IGNITION-MATH_DOXYGEN_TAGFILE} = ${IGNITION-MATH_API_URL}"
   "${IGNITION-MSGS_DOXYGEN_TAGFILE} = ${IGNITION-MSGS_API_URL}"
   "${IGNITION-PHYSICS_DOXYGEN_TAGFILE} = ${IGNITION-PHYSICS_API_URL}"
   "${IGNITION-PLUGIN_DOXYGEN_TAGFILE} = ${IGNITION-PLUGIN_API_URL}"
   "${IGNITION-TRANSPORT_DOXYGEN_TAGFILE} = ${IGNITION-TRANSPORT_API_URL}"
   "${IGNITION-SENSORS_DOXYGEN_TAGFILE} = ${IGNITION-SENSORS_API_URL}"
   "${IGNITION-COMMON_DOXYGEN_TAGFILE} = ${IGNITION-COMMON_API_URL}"
)

Resolves issue #98

Signed-off-by: Nate Koenig [email protected]

[chapulina]

This is great! 👍